1. Field of the Invention
The present invention relates to an apparatus and method for detecting very small distances, and more particularly to proximity sensing.
2. Related Art
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching several nanometers.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of lithography systems. In this context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants, cause minute temperature changes, or come in contact with the work-surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in lithography systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers are made of or contain deposits of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP). In these cases, capacitance and optical gauges may provide spurious results.
Air gauge sensors typically emit a dehydrated, filtered air flow onto a surface (e.g., the silicon wafer) and then measure its back pressure to determine distance between the measurement nozzle and that surface. More sensitive air gauge sensors use both reference and measurement nozzles emitting an air flow onto reference and measurement surfaces to determine surface distances. An air gauge sensor is not vulnerable to concentrations of electric charges nor electrical, optical and other physical properties of the wafer's surface. Current semiconductor manufacturing techniques, however, require that proximity is gauged with high precision of the order of nanometers. Earlier versions of air gauge sensors, unfortunately, often do not meet today's lithography requirements for precision. Today's requirements for nanometer repeatability and registration accuracy are more stringent than what is currently available in the industry at large. Additionally, earlier devices do not meet today's needs for dimensional stability throughout specific temperature ranges.
What are needed are systems and methods for providing precise, nanometer-scale measurements by a gas gauge proximity sensor that also exhibits dimensional stability.